The invention resides in an apparatus for determining the moisture content of a medium, which forms the dielectric of a measuring conduit representing a probe. The apparatus includes an electronic evaluation device with a pulse signal generator supplying a pulse signal to one end of the measuring conduit, a receiver receiving the reflected signal at the same end and a device for determining the time delay between the sent signal the reflected signal.
Such an apparatus is known for example from EP 0 478 851 B1 and has been successfully commercialized by the patentee. The essential element of the known apparatus is the measuring cable.
The measuring conduit can have different forms. The measuring conduit may consist for example of two parallel rods or it may be in the form of a probe as disclosed in DE 43 34 649 C2.
The probe disclosed in DE 43 34 649 C2 comprises a cylindrical body on which the measuring conduit is disposed. The probe body may also be shaped differently depending on requirements. For example, the body may be triangular in cross-section, square, hexagonal or oval. The body may furthermore be solid or hollow. However, it always includes on its surface electrical conductors arranged in spaced relationship and forming the measuring conduit.
In the known apparatus, the probe is connected to the electronic evaluation unit by way of a coaxial cable. With the use of a co-axial cable as connecting means, errors introduced by the connecting means can be kept relatively small. But, it is still desirable to increase the accuracy of the apparatus.
It is the object of the present invention to provide an apparatus for determining the moisture content of a medium which apparatus has a higher accuracy than those referred to above.
In an apparatus for determining the moisture content of a medium forming a dielectric of a measuring conduit of a probe, which apparatus includes an electronic evaluation unit, a signal generator providing a pulse signal to one end of the measuring conduit, a receiver for receiving a reflected signal returned to the one end of the measuring conduit and a time measuring device for determining the time elapsed between the supply of the signal and the return of the reflected signal, the electronic evaluation unit is arranged in close proximity to the one end of the measuring conduit and provided with temperature compensating means.
Since the electronic evaluation unit includes an arrangement for temperature compensation and is arranged directly at the end of the measuring conduit a substantially greater accuracy can be achieved. Although, in the past, an arrangement of the evaluation unit at the end of the measuring conduit and a temperature compensation are provided only for a relatively small accuracy improvement, it has surprisingly been found that a combination of a temperature compensation with the arrangement of the electronic evaluation unit directly at the end of the measuring conduit provides for a substantial improvement of the accuracy of the apparatus. The systematic error inherent in the known apparatus could be reduced with the arrangement according to the invention essentially to zero.
An embodiment of the invention, wherein the electronic evaluation unit is disposed in a probe housing has been found to be particularly advantageous. With the arrangement of the electronic evaluation unit within the probe housing, the electronic evaluation unit is not only arranged directly at the end of the measuring conduit but, furthermore, the apparatus is very compact and also very easy to use.
It is in this connection particularly advantageous, if the temperature compensation means is also arranged in the probe housing. In this way, the temperature of the measuring conduit can be compensated for together with the temperature of the electronic evaluation unit.
It is very advantageous if the temperature compensation means includes a storage, in which a first reference measurement value of a predetermined material moisture content at a first temperature of the apparatus or, respectively, the electronic evaluation unit and a second measurement reference value of the predetermined material moisture content at a second temperature of the apparatus or, respectively, the electronic evaluation unit is stored. It is particularly advantageous if the reference measurement values are determined individually for each apparatus manufactured. With the reference measurement values, the changes of a measurement value over the temperature between the first and second temperature can easily be determined by interpolation. When, during later operation, a moisture content value determined in a measurement is corrected with the temperature-dependent adjustments determined in this way, the measurement value becomes highly accurate.
An embodiment of the invention wherein the measurement conduit is arranged at the surface of the probe housing and, in this area, the surface includes a pillow filled with gel has also been found to be advantageous. Then the measuring conduit can be very easily placed onto the medium to be measured. With the gel-filled pillow the probe can be placed onto the surface of the medium to be measured in a very gentle manner.
In another embodiment of the invention, electric power is supplied by an energy storage device also arranged in the probe housing. In this way, handling of the apparatus is further substantially facilitated.
It has also been found to be advantageous if the probe housing includes indicating means. With such indicating means in the probe housing use of the apparatus is facilitated.
Furthermore, the probe housing may include a handle with switch buttons for starting a measurement procedure. Then the apparatus can be handled with one hand.
Further particulars, features and advantages of the present invention will be described below on the basis of the accompanying drawing.